Hi,
The internal doc describes TARGET_MODES_TIEABLE_P as:
This hook returns true if a value of mode MODE1 is accessible in
mode MODE2 without copying.
If ‘TARGET_HARD_REGNO_MODE_OK (R, MODE1)’ and
‘TARGET_HARD_REGNO_MODE_OK (R, MODE2)’ are always the same for any
R, then ‘TARGET_MODES_TIEABLE_P (MODE1, MODE2)’ should be true. If
they differ for any R, you should define this hook to return false
unless some other mechanism ensures the accessibility of the value
^^^^^^^^^^^^^^^
in a narrower mode.
You should define this hook to return true in as many cases as
possible since doing so will allow GCC to perform better register
allocation. The default definition returns true unconditionally.
I'm wondering what is the "other mechanism" mentioned here...
--
Xi Ruoyao <[email protected]>
